The 2,5-dimethyl-2,4-hexadiene, prepared in accordance with the present invention, is a useful intermediate in the preparation of other organic compounds. It is especially useful as an intermediate in the synthesis of chrysanthemum monocarboxylic acid which is of importance in the manufacture of certain insecticides.
A number of methods for preparing 2,5-dimethyl-2,4-hexadiene are known. Among these known methods for preparing the diene are those which involve the use of 2,5-dimethyl-2,5-hexanediol, the starting compound used in the method of the present invention. Previous workers have reported various methods for converting the diol to the 2,4-hexadiene. These known methods, however, have certain deficiencies. Although the diol can be converted to the diene by heating with alumina, high temperatures are required and the product is contaminated with undesirable C.sub.8 H.sub.14 isomers. The dehydration has been accomplished at somewhat lower temperatures by passing preheated vapors of the diol over a specially prepared catalyst that is obtained by heating alumina with phosphoric acid. Alternatively, a lower temperature acid catalyzed dehydration of the diol can be carried out in a slow reaction, but considerable amounts of undesired by-products are also formed. When the dehydration of the diol is carried out at high temperatures in the presence of a low acidity alumina and a lower aliphatic alcohol, the principal product is the 1,5-hexadiene rather than the 1,4-hexadiene.
It has now surprisingly been found, in accordance with the present invention, that under particular conditions 2,5-dimethyl-2,5-hexanediol can be converted to 2,5-dimethyl-2,4-hexadiene with good yields and selectivity. This method uses a different catalyst than those previously employed and when the reaction is carried out in the presence of a lower aliphatic alcohol, the desired 1,4-hexadiene is obtained in high yields.
The method of the present invention is accomplished at moderate temperatures in the presence of a simple catalyst and can be achieved rapidly in a continuous process. It also avoids the strong acidic conditions of various prior processes.